A common wind measurement lidar emits single-frequency continuous-wave light (laser beam) called a local beam into the air, first, and receives a scattered beam from aerosol. Then, it obtains a received signal through heterodyne detection between the laser beam and the scattered beam. The received signal is passed through FFT processing to obtain its peak frequency from which a frequency shift amount due to the movement of the aerosol is calculated. From the frequency shift amount, it measures the wind speed (the degree of turbulence of the wind field) in the direction of the laser emission. At this time, the level of a noise spectrum at the time of the frequency shift analysis varies owing to power fluctuation of the laser beam, or temperature fluctuation of an optical receiver that carries out the heterodyne detection. In view of this, a Patent Document 1, for example, describes a removal method of the noise offset.
A wind measurement lidar disclosed in the Patent Document 1 stores a signal spectrum in advance, which is acquired in a state without emitting a laser beam, as a noise spectrum. Then, taking a frequency difference of the noise spectrum from the signal spectrum acquired in the state without emitting the laser beam, followed by making an offset correction, it detects a frequency peak position of the signal spectrum, and obtains the frequency shift amount.